As one of optical modulators for modulating laser light, LiNbO3 modulators (hereinafter, referred to as “LN modulator”) are widely used. The LN modulator is one of Mach-Zehnder optical modulators (hereinafter, referred to as “MZ optical modulator”). Such an LN modulator outputs light, for example, as follows. That is, the LN modulator generates output light by (1) branching input laser light so that branched laser lights enter respective two waveguides to which respective voltages, whose directions are opposite to each other, are applied, (2) causing (i) laser light to have a phase advance of amount φ in a first waveguide and (ii) laser light to have a phase delay of amount φ in a second waveguide, and (3) interferometrically combining the laser light (hereinafter, referred to as “advance light”) whose phase has been advanced by the amount φ through the first waveguide and the laser light (hereinafter, referred to as “delay light”) whose phase has been delayed by the amount φ through the second waveguide.
The amount φ by which the phase is advanced or delayed in the LN modulator is determined in accordance with an externally applied driving voltage V. In a case where a predetermined voltage V0, which causes the advance light and the delay light to have reversed phases, is applied as the driving voltage V, the advance light and the delay light destructively interfere with each other, so that intensity of output light (i.e., electric power of output light) is minimized. On the other hand, in a case where a predetermined voltage V1, which causes the advance light and the delay light to have identical phases, is applied as the driving voltage V, the advance light and the delay light constructively interfere with each other, so that intensity of output light is maximized. In view of this, it is possible to generate an optical signal, whose intensity has been modulated in accordance with an input signal, by applying (i) a driving voltage V (=V0) when a value of the input signal (data signal) is 0 or (ii) a driving voltage V (=V1) when the value of the input signal is 1.
It is known that the LN modulator has a problem of an operating point drift. Note that the “operating point drift” means a phenomenon in which an applied voltage V, which causes output light to have minimum or maximum intensity, is shifted from the predetermined voltage V0 or V1 due to a change with time or a disturbance. The disturbance causing the operating point drift is typically a temperature change.
Patent Literature 1 discloses a well known compensation method for compensating an operating point drift. FIG. 7 illustrates a configuration of an optical modulation system 100 disclosed in Patent Literature 1. The optical modulation system 100 includes a light source 111, a driving circuit 112, an LN modulator (external modulator) 113, a low frequency oscillator 114, a low frequency signal superimposing means 115, a low frequency signal detection means 116, and a control means 117 (see FIG. 7).
According to the optical modulation system 100, the LN modulator 113 serves as an external modulator for modulating, in accordance with an input signal #1, intensity of laser light emitted by the light source 111. Note, however, that the driving circuit 112 applies, to the LN modulator 113, a driving voltage V which is not determined in accordance with a value of the input signal #1 itself but is determined in accordance with a value of an input signal #3 whose amplitude has been modulated in accordance with a low frequency signal #2. The low frequency oscillator 114 is provided so as to generate the low frequency signal #2. The low frequency signal superimposing means 115 is provided so as to modulate an amplitude of the input signal #1 in accordance with the low frequency signal #2. Here, the low frequency signal #2 may be a signal (such as that of a sine wave, a rectangular wave, or a triangular wave) having a frequency f0 which is sufficiently lower than a frequency of the input signal #1.
In a case where the driving voltage V, which is determined in accordance with the value of the input signal #3 whose amplitude has been modulated in accordance with the low frequency signal #2, is thus applied to the LN modulator 113, the LN modulator 113 can be considered to output an optical signal L having the following features: That is, in a case where no operating point drift occurs, the optical signal L outputted by the LN modulator 113 contains a low-frequency component having a frequency twice as high as the frequency f0 of the low frequency signal #2. On the other hand, in a case where an operating point drift occurs, the optical signal L outputted by the LN modulator 113 contains a low-frequency component having a frequency identical with the frequency f0 of the low frequency signal #2. In this case, a phase difference between (i) the low-frequency component (having the frequency identical with the frequency f0) contained in the optical signal L and (ii) the low frequency signal #2 becomes 0 or Π depending on a direction of the operating point drift.
The low frequency signal detection means 116 detects a low-frequency component, which has the frequency identical with the frequency f0, from the optical signal L (more properly, a voltage signal obtained by carrying out a photoelectric conversion and a current-voltage conversion with respect to the optical signal L) outputted by the LN modulator 113. Moreover, the low frequency signal detection means 116 compares a phase of a detected low-frequency component with a phase of the low frequency signal #2 so as to specify a direction of the operating point drift. The control means 117 supplies, to the driving circuit 112, a control signal #4 for changing an operating point of the driving circuit 112 in accordance with the direction of the operating point drift specified by the low frequency signal detection means 116. The driving circuit 112 changes an applied voltage V, which is to be applied to the LN modulator 113, based on the control signal #4 supplied from the control means 117. Specifically, in a case where the value of the input signal #1 is 0, the driving circuit 112 changes the applied voltage V from the predetermined voltage V0 to a voltage V0±dV. Alternatively, in a case where the value of the input signal #1 is 1, the driving circuit 112 changes the applied voltage V from the predetermined voltage V1 to a voltage V1±dV.
The optical modulation system 100 compensates an operating point drift by carrying out such feedback control. This makes it possible to achieve a stable feedback control and to generate a stable optical signal.
Delay interferometers are widely used as an optical demodulator for demodulating an optical signal whose phase has been modulated. The delay interferometer generates output light by (1) branching input signal light so that branched signal lights enter respective two waveguides having different lengths and (2) interferometrically combine (i) first signal light, which has passed through a first waveguide (i.e., a longer waveguide), and (ii) second signal light, which has passed through a second waveguide (i.e., a shorter waveguide).
A state of the output light of the delay interferometer is determined based on a phase difference between the first signal light and the second signal light. In a case where the lengths of the first and second waveguides are set so that the first signal light is delayed with respect to the second signal light by one (1) symbol, it is possible to obtain output light having a state determined in accordance with a phase difference (hereinafter, referred to as “inter-symbol phase difference”) between signal light (i.e., first signal light) corresponding to a previous symbol and signal light (i.e., second signal light) corresponding to a current symbol.
An optical coupler having two output ports is used to interferometrically combine the first signal light and the second signal light. A demodulated signal is obtained by differentially amplifying current signals, which have been obtained by photoelectrically converting optical signals outputted from the respective two output ports, with the use of a transimpedance amplifier. In a case where the inter-symbol phase difference is 0, a value of the demodulated signal is maximized. In a case where the inter-symbol phase difference is n, a value of the demodulated signal is minimized.
A DBPSK (optical differential binary phase shift keying) demodulator includes a single delay interferometer. In the DBPSK demodulator, an optical signal, which has been subjected to a DBPSK modulation, is converted to the intensity modulated signal by the delay interferometer. Then, the optical signal, whose intensity has been modulated, is subjected to a photoelectric conversion so that a demodulated signal is obtained. A DQPSK (optical differential quadrature phase shift keying) demodulator includes two delay interferometers having different delay amounts. In the DQPSK demodulator, each of the delay interferometers converts an optical signal, which has been subjected to a DQPSK modulation, to the two intensity modulated signals. Then, the two optical signals are subjected to a photoelectric conversion so that an I-channel demodulated signal and a Q-channel demodulated signal are obtained. Note that details of the DQPSK demodulator are disclosed in, for example, Patent Literature 2.
The delay interferometer also has a problem similar to an operating point drift in the LN modulator. That is, in a case where a wavelength of signal light or a temperature of the delay interferometer is changed, an inter-symbol phase difference is shifted from a predetermined value. This phenomenon is called “phase drift.”
In order to compensate such a phase drift, a low frequency signal can be used. Specifically, such a phase drift can be compensated by (1) slightly oscillating a delay amount, by which signal light that has entered the first waveguide is delayed with respect to signal light that has entered the second waveguide, with the use of a low frequency signal and (2) increasing or decreasing the delay amount in accordance with a phase difference between the low frequency signal and a low-frequency component contained in a demodulated signal. Note that the increasing or decreasing of the delay amount can be carried out by, for example, raising or reducing a temperature of a medium constituting the first waveguide or the second waveguide with the use of a heater.
Citation List
Patent Literatures
Patent Literature 1
Japanese Patent Application Publication Tokukaihei No. 3-251815 A (Publication date: Nov. 11, 1991)
Patent Literature 2
Japanese Translation of PCT International Application Tokuhyo No. 2004-516743 A (Publication date: Jun. 3, 2004)